This invention relates to a zinc containing aqueous solution that resists zinc precipitation. More particularly, silicon is added to an alkaline, aqueous electrolyte that contains zinc ions in a specified zinc to silicon ratio.
Alkaline aqueous chemical solutions containing zinc are widely used such as for galvanizing steel and providing tarnish resistance to copper and copper alloys. One limitation with these alkaline solutions is that zinc continuously precipitates from the solution, both during use and while the solution is not in use.
To maintain the chemical solution in optimum operating condition, the solution must be filtered to remove precipitated zinc metal, frequently analyzed to determine the present zinc content and replenished to replace the precipitated zinc. These steps are time consuming and the chemical solution is out of service during the filtering, analyzing, replenishing sequence. U.S. Pat. No. 5,098,796 to Lin et al. discloses an electrolyte for codepositing a mixture of chromium and zinc on a metallic substrate that is an alkaline, aqueous zinc containing solution. The codeposited layer imparts the substrate with tarnish resistance at room temperature and at elevated temperatures, up to about 220.degree. C. The codeposited layer also improves the adhesion of the substrate to a polymer adhesive or to a polymer molding resin.
The present inventor has now discovered that when the zinc content of this electrolyte drops below 0.8 grams per liter (g/l), adhesion of the anti-tarnish coating to a copper substrate is impaired.
Therefore, to eliminate lost time while the chemical solution is being filtered, analyzed and replenished and to maintain the enhanced adhesion of a codeposited layer of chromium and zinc, there is a need for an alkaline zinc containing electrolyte that resists zinc precipitation.